Time filter

Source Type

Vilafranca del Penedès, Spain

Chen Y.-P.,CAS Institute of High Energy Physics | Zhang S.,CAS Institute of High Energy Physics | Torres D.F.,Catalan Institution for Research and Advanced Studies | Zhang S.-N.,CAS Institute of High Energy Physics | And 4 more authors.
Astronomy and Astrophysics | Year: 2011

Context. The 2008 outburst of the atoll source IGR J17473-2721 was observed by INTEGRAL, RXTE and Swift. Tens of type-I X-ray bursts were found in this outburst. Aims. Joint observations by INTEGRAL, RXTE, and Swift provide sufficient data to look into the behavior of IGR J17473-2721 at the rising part of the 2008 outburst. The relation between the duration of the bursts and the accretion rate and the nature of the corona producing the observed power-law component can therefore be studied in detail. Methods. We analyze observational data of IGR J17473-2721, focusing on the spectral evolution during the state transition from quiescent to low hard state (LHS), and on the flux dependence of the type-I X-ray bursts along the outburst. Results. We find that the joint INTEGRAL, RXTE and Swift energy spectrum can be well fitted with a model composed of a blackbody and a cutoff power-law, with a cutoff energy decreasing from ∼150 keV to ∼40 keV as the source leaves the quiescent state toward the low hard state. This fits into a scenario in which the corona is cooled by the soft X-rays along the outburst evolution, as observed in several other atoll sources. Fifty-seven type-I bursts were reported in the 2008 outburst of IGR J17473-2721. By using the flux measured in the 1.5-30 keV band, we find that the linear relationship between the burst duration and the flux still holds for those bursts that occur at the decaying part of the low hard state, but with a different slope than the overall one that was estimated with the bursts happening in the whole extent of, and for the rest of the low hard state. The significance of such a dichotomy in the type-I X-ray bursts is ∼3σ under an F-test. Similar results are hinted at as well with the broader energy-band that was adopted recently. This dichotomy may be understood in a scenario where part of the accreting material forms a corona on the way of falling onto the surface of the neutron star during the decaying part of the low hard state.Based on the accretion rates of the preceding LHS, estimated from type-I X-ray bursts and from persistent emission, at least for IGR J17473-2721, most of the accretion material may fall on the neutron star (NS) surface in the LHS. Considering the burst behavior in the context of the outburst indicates a corona formed on top of the disk rather than on the NS surface. © 2011 ESO. Source

Degenaar N.,University of Amsterdam | Degenaar N.,University of Michigan | Wijnands R.,University of Amsterdam | Cackett E.M.,University of Cambridge | And 5 more authors.
Astronomy and Astrophysics | Year: 2012

We report on the results of a four-year long X-ray monitoring campaign of the central 1.2 square degrees of our Galaxy, performed with Chandra and XMM-Newton between 2005 and 2008. Our study focuses on the properties of transient X-ray sources that reach 2-10 keV luminosities of L X ≳ 10 34 erg s -1 for an assumed distance of 8 kpc. There are 17 known X-ray transients within the field of view of our campaign, eight of which were detected in outburst during our observations: the transient neutron star low-mass X-ray binaries GRS 1741-2853, AX J1745.6-2901, SAX J1747.0-2853, KS 1741-293 (all four are also known X-ray bursters), and GRO J1744-28 (a 2.1 Hz X-ray pulsar), and the unclassified X-ray transients XMM J174457-2850.3, CXOGC J174535.5-290124 and CXOGC J174541.0-290014. We present their X-ray spectra and flux evolution during our campaign, and discuss our results in light of their historic activity. Our main results include the detection of two thermonuclear X-ray bursts from SAX J1747.0-2853 that were separated by an unusually short time interval of 3.8 min. Investigation of the lightcurves of AX J1745.6-2901 revealed one thermonuclear X-ray burst and a ∼1600-s long X-ray eclipse. We found that both XMM J174457-2850.3 and GRO J1744-28 displayed weak X-ray activity above their quiescent levels at L X ∼ 10 33-34 erg s -1, which is indicative of low-level accretion. We compare this kind of activity with the behaviour of low-luminosity X-ray transients that display 2-10 keV peak luminosities of L X ∼ 10 34 erg s -1 and have never been seen to become brighter. In addition to the eight known X-ray transients, we discovered a previously unknown X-ray source that we designate XMMU J174654.1-291542. This object emits most of its photons above 2 keV and appears to be persistent at a luminosity of L X ∼ 10 34 erg s -1, although it exhibits strong spectral variability on a time scale of months. Based on its X-ray properties and the possible association with an infrared source, we tentatively classify this object as a cataclysmic variable. No new transients were found during our campaign, reinforcing the conclusion of previous authors that most X-ray transients recurring on a time scale of less than a decade have now been identified near the Galactic centre. © ESO, 2012. Source

Ji L.,CAS Institute of High Energy Physics | Zhang S.,CAS Institute of High Energy Physics | Chen Y.,CAS Institute of High Energy Physics | Zhang S.-N.,CAS Institute of High Energy Physics | And 7 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

To investigate the possible cooling of the corona by soft X-rays bursts, we have studied 114 bursts embedded in the known X-ray evolution of 4U 1636-536. We have grouped these bursts according to the ratio of the flux in the 1.5-12 keV band with respect to that in the 15- 50 keV band, as monitored by Rossi X-ray Timing Explorer/All Sky Monitor and Swift/Burst Alert Telescope, respectively.We have detected a shortage at hard X-rays while bursting. This provides hints for a corona cooling process driven by soft X-rays fed by the bursts that occurred on the surface of neutron star. The flux shortage at 30-50 keV has a time lag of 2.4 ± 1.5 s with respect to that at 2-10 keV, which is comparable to that of 0.7 ± 0.5 s reported in bursts of IGR 17473-2721. We comment on the possible origin of these phenomena and on the implications for the models on the location of the corona. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Savolainen P.,Metsahovi Radio Observatory | Hannikainen D.C.,Metsahovi Radio Observatory | Hannikainen D.C.,University of Turku | Paizis A.,National institute for astrophysics | And 3 more authors.
AIP Conference Proceedings | Year: 2010

We present long-term spectral and timing results from an INTEGRAL monitoring program of persistently bright neutron star Low-Mass X-ray Binaries, i.e. the three bright Atoll sources GX3+1, GX9+1 and GX9+9, and the Z sources GX5-1, GX17+2, GX340+0 and GX349+2. From the available observing periods between 2003 and 2009, each lasting ∼2 months, we have selected a few sample periods for each source, and analyzed all JEM-X and IBIS/ISGRI data with offsets <4 degrees. We seek an explanation for the dichotomy between the hard X-ray tails or lack thereof in the (otherwise very similar) X-ray spectra of Z sources and bright Atolls, respectively. © 2010 American Institute of Physics. Source

Savolainen P.,Metsahovi Radio Observatory | Paizis A.,National institute for astrophysics | Farinelli R.,University of Ferrara | Hannikainen D.C.,Metsahovi Radio Observatory | And 3 more authors.
AIP Conference Proceedings | Year: 2011

We have investigated the spectral and temporal evolution of Neutron Star Low-Mass X-ray Binaries (NS LMXBs) using a sample of eight bright sources. INTEGRAL spectra from 2003 to 2009 and some near-simultaneous RXTE and Swift spectra have been fitted with the recently developed thermal and bulk Comptonization model compTB. In this proposed scenario, the transient hard X-ray tails exhibited by the Z sources and GX 13+1 are connected to efficient bulk motion Comptonization in the inner Transition Layer (TL), while the lack of a tail could be due to too low or too high local accretion rates. Another difference between the source types - the long-term modulation of the Atoll source light curves (with the notable exception of GX 13+1) - is seen to be mostly independent of spectral evolution, while presumably driven by changes in the overall accretion rate from the companion. © 2011 American Institute of Physics. Source

Discover hidden collaborations